Ever since Pedersen's discovery that easy-to-make crown ethers provide size-selective binding to alkali cations, macrocycles have proven to be fundamental to the foundation of molecular recognition and have seeded potential applications across chemistry and biology, such as phase transfer catalysis and drug delivery. Building on their propensity for self-assembly, macrocycles also serve as precursors to interlocked molecules where host-guest complexes are captured covalently as rotaxanes and catenanes using mechanical bonding. While this diversity of usage marks macrocycles as singularly attractive synthetic and functional targets, their plentiful numbers and varieties demands that any new macrocycle deliver a distinctive cross section of supramolecular properties.
Moreover, with macrocycles serving as central precursors, the yield and the scale of their production are critical to their early assessment. Work by Busch demonstrated the power of pre-organization, whether using templates or favorable intramolecular contacts, to realize high-yielding macrocyclizations. While these principles promise macrocycles in one-pot reactions from simple building blocks that can be conducted on large scales, realization of this potential is rare. Rather, multistep reaction schemes conducted at modest scales are typical. Nevertheless, a number of macrocycles are intrinsically easy to prepare, e.g., symmetric tetraphenylporphyrins and recently Ogoshi's pillar[5]arenes, with some that can be produced on gram scales, e.g., Gong's oligoamides. Others achieve this outcome through deliberate investigation, such as Moore's production of shape-persistent arylene ethynylene macrocycles using reversible alkyne metathesis under thermodynamic control.
There are only a few semi-planar, C5-symmetric macrocycles. Pentameric phenylene ethynylenes have been examined for surface self-assembly at the liquid-solid interface. Zeng's aryl-amide pentamers can be optimized for one-pot preparations, and can be tailored for selective cation binding, dense crystal packing, and gelation. MacLachlan's aryl-imine campestarene, which can be prepared in high yields in one pot, shows keto-enol tautomerism within the H-bonded imines.